1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device having an impurity diffusion layer, in particular, to a method of manufacturing a semiconductor device having shallow junctions.
2. Description of the Related Art
FIG. 3A and FIG. 3B show an example of prior art ion doping technologies. In this method, ions are directly doped to a substrate with an accelerating voltage of 5 to 80 keV. Thereafter, the substrate is annealed in a nitrogen atmosphere containing oxygen in the state that the front surface thereof is exposed so as to form a diffusion layer.
However, in such known methods, unnecessary impurities other than desired impurities would be doped to the substrate. In addition, when the ion doping process and the annealing process are performed, the front surface of the silicon substrate may be damaged.
To solve such a problem, a silicon oxide film is formed on the substrate. Ions are doped through the silicon oxide film (hereinafter, such an oxide film may be referred to as through-oxide film). This method has been disclosed in for example Japanese Patent Laid-Open Publication No. 58-96763. FIG. 4A and 4B show a practical method for forming a through-oxide film. In this method, a silicon oxide film with a thickness of 100 nm or more is formed on the front surface of a substrate. Ions are doped to the substrate through the silicon oxide film at an accelerating voltage of 40 keV. Thereafter, with the silicon oxide film left, the substrate is annealed in a nitrogen atmosphere. Alternatively, after the silicon oxide film is removed and then another silicon oxide film is formed, the substrate is annealed in a nitrogen atmosphere. Thus, a diffusion layer is formed on the substrate. In this method, unnecessary impurities other than desired impurities can be prevented from doped in the substrate.
In the above-described methods, the sheet resistance rises. Intensive studies by the inventor of the present invention show that impurities concentrate in an oxide film. Thus, the concentration of impurities in the diffusion layer decreases. In the silicon oxide film forming method, since doped impurities concentrate in the silicon oxide film, the concentration of impurities of the diffusion layer decreases. Likewise, in the direct ion doping method of which ions are directly doped to a substrate without a silicon oxide film, the substrate is annealed in a nitrogen atmosphere containing several % of oxide so as to prevent the front surface of the substrate from roughening. While the substrate is being annealed, since an oxide film is formed on the front surface of the substrate, impurities concentrate in the oxide film as with the silicon oxide film forming method. Thus, the concentration of the impurities in the diffusion layer decreases. Because of such reasons, in the conventional methods, the sheet resistance increases.
In addition, as semiconductor devices are finely structured, junctions are more shallowly diffused. To do that, it is necessary to lower the accelerating voltage of ions, in reality, 3 keV or less. However, it is difficult to sufficiently shallowly diffuse ions in the method of which ions are doped through the through-oxide film. In addition, the sheet resistance remarkably increases. Table 1 shows the relation between the presence/absence of oxide film and sheet resistance. From the table, it is clear that the doping energy of boron is inversely proportional to the sheet resistance of the through-oxide film.
TABLE 1 Presence/Absence Absence Presence Presence Absence of through-oxide 2.5 nm 50 nm film in ion doping process Presence/Absence Absence Presence Presence Presence of cover-oxide (leave (leave (deposited film in annealing through- through- by 100 nm process oxide film oxide film after as it is) as it is) removal of - oxide film) Annealing Nitrogen Nitrogen Nitrogen Nitrogen atmosphere atmosphere atmosphere atmosphere atmos- containing containing containing phere 100 ppm or oxygen (*1) oxygen containing less of oxygen oxygen Boron doping condition 0.5 keV 63.7 746.9 2373.0 675.3 5 .times. 1014 cm-2 1 keV 311.5 340.5 464.8 5 .times. 1014 cm-2 2 keV 232.7 231.7 258.3 5 .times. 1014 cm-2 5 keV 102.0 99.3 108.9 5 .times. 1014 cm-2 Values in the table represent average values of sheet resistance values (.OMEGA./.quadrature.) at 49 points on the wafer. (*1) "Nitrogen atmosphere containing oxygen" means a nitrogen atmosphere containing 100 ppm or more of oxygen.
In a doping process with ions of boron or ions of a compound thereof, when a silicon substrate is annealed with a silicon oxide film, ions of boron are more deeply diffused. Thus, junctions are more deeply diffused than designed values. Hereinafter, this phenomenon is referred to as "accelerating diffusion". Recent studies show that the accelerating diffusion takes place as a peculiar phenomenon upon doping of ions of boron or ions of a compound thereof.
An object of the present invention is to solve the above-mentioned problems contained in the prior art.